In general, thermal recording materials that provide recorded images utilizing a color development reaction between a colorless or pale basic leucodye and a developer due to heat have been widely put to practical use for facsimile, computer field, various measurement equipments and the like, in view of advantages such as highly vivid color development, recording without noise, comparatively economical and compact apparatuses, easy maintenance and the like. In recent years, use as an output medium for various printers and plotters such as label, ticket, compact mobile terminal for outdoor measurement (handy terminal), carriage slip and the like has been growing rapidly. Particularly, use as an output medium for inspection of electricity, gas, tap water and the like, mobile printer (handy terminal) used for in-house sales (bullet train etc.), custody control in warehouses and the like is markedly increasing. Such mobile printers (handy terminals) have been downsized for easy portability, thus saving printing energy and driving energy. Along therewith, thermal recording materials are required to have high quality to meet higher color development sensitivity and printing adequacy comparable to that of general printing (offset lithography etc.). Moreover, since they are frequently used outside, preservation property to stand use in a harsh environment as compared to conventional ones, such as daylight, being left at midsummer high temperature inside a car, exposure to moisture such as rain etc., and the like is being required (i.e., resistance of a thermal recording material to heat, humidity, water and the like, which makes it possible to maintain density of the recorded images and suppress color development in non-image areas even when exposed to heat, humidity, water and the like).
To improve preservation property, for example, patent document 1 proposes addition of an antioxidant to a thermal recording layer together with a developer. However, the addition is not preferable since image quality such as color development sensitivity and the like is degraded. To improve preservation property, moreover, a developer having high preservation property such as ureaurethane compound disclosed in patent document 2 or patent document 3, phenolic compounds such as diaphenylsulfone crosslinking type compounds disclosed in patent document 4 and the like may be used alone. However, such developers having high preservation property are more expensive than general-purpose developers, and generally show low color development sensitivity even though the preservation property is improved to a certain level. As such, a thermal recording material having sufficiently satisfactory property for practical use in terms of color development sensitivity, preservation property (heat resistance, moisture resistance, water resistance etc.) and cost has not been obtained yet.    patent document 1: JP-A-59-2891    patent document 2: WO00/14058    patent document 3: JP-A-2002-332271    patent document 4: WO97/16420